IMU: Difference between revisions

An '''Inertial Measurement Unit''' ('''IMU''') is an electronic [[sensor]] [[device]] that measures and reports a body's specific force, angular rate, and sometimes the orientation of the body, using a combination of [[accelerometer]]s, [[gyroscope]]s, and often [[magnetometer]]s.<ref name="TDK_IMU_Overview"> TDK InvenSense. What is an Inertial Measurement Unit (IMU)? [https://invensense.tdk.com/technology/motion/imu/ TDK InvenSense Website]. Accessed October 26, 2023.</ref> IMUs are fundamental components in [[virtual reality|Virtual Reality (VR)]] and [[augmented reality|Augmented Reality (AR)]] systems for tracking the orientation of [[Head-Mounted Display|HMDs]] and [[Input Devices]] like controllers.

A typical IMU integrates multiple sensor types onto a microchip:

When an IMU includes all three sensors (accelerometer, gyroscope, and magnetometer), it is sometimes referred to as a 9-axis IMU or a [[MARG]] (Magnetic, Angular Rate, and Gravity) sensor.<ref>Madgwick, Sebastian OH, Andrew JL Harrison, and Ravi Vaidyanathan. "Estimation of IMU and MARG orientation using a gradient descent algorithm." IEEE international conference on rehabilitation robotics. IEEE, 2011.</ref>

Raw data from individual sensors can be noisy (e.g., accelerometers during fast movement) and inaccurate (e.g., gyroscopes drift). [[Sensor fusion]] algorithms, such as [[Kalman filter]]s or complementary filters, are essential.<ref name="Mahony_Filter"> Mahony, R., Hamel, T., & Pflimlin, J. M. (2008). Nonlinear Complementary Filters on the Special Orthogonal Group. IEEE Transactions on Automatic Control, 53(5), 1203–1218. [https://ieeexplore.ieee.org/document/4532514 Abstract Link]</ref> These algorithms intelligently combine the data from the accelerometers, gyroscopes (and magnetometers, if present) to produce a single, more accurate, stable, and low-latency estimate of the device's orientation in real-time.

IMUs provide the rapid [[orientation tracking]] needed to update the virtual view in sync with the user's head movements. This low latency is critical for user comfort. The typical update rate of modern IMUs used in VR headsets is between 500Hz to 1000Hz, much faster than most visual tracking systems can achieve alone.<ref>Niehorster, Diederick C., Li Li, and Markus Lappe. "The accuracy and precision of position and orientation tracking in the HTC Vive virtual reality system for scientific research." i-Perception 8.3 (2017).</ref>

An IMU inherently provides [[Degrees of Freedom|3 DoF tracking]], measuring orientation changes (pitch, yaw, roll). This is sufficient for basic VR experiences like 360-degree video viewing on mobile VR headsets where the user's physical position in the room is not tracked.

For full [[6DoF]] tracking (which includes [[positional tracking]] – translation along X, Y, and Z axes), IMU data is combined via sensor fusion with data from other tracking systems. These can include:

*  [[Inside-out tracking]]: Cameras on the HMD observe the external environment.

*  [[Zero velocity updates]] (ZUPTs) during periods of detected stillness to reset velocity error accumulation<ref>Cadena, Cesar, et al. "Past, present, and future of simultaneous localization and mapping: Toward the robust-perception age." IEEE Transactions on robotics 32.6 (2016): 1309-1332.</ref>

For optimal performance in VR/AR applications, IMUs typically require:

*  Integrated processing capabilities (sometimes including basic sensor fusion)<ref>Angelini, Lee, et al. "Understanding sensors: prioritizations for selecting sensors in mobile VR applications." Internet Research (2022).</ref>

Next-generation IMUs for VR/AR are focusing on:

*  Further reduction in sensor noise and drift characteristics<ref>Adams, Michael D. "MEMS IMU Navigation with Model Based Dead-Reckoning and One-Way-Travel-Time Acoustic Measurements." IEEE Journal of Oceanic Engineering (2023).</ref>

Several companies manufacture IMUs used in consumer electronics, including VR/AR devices:

*  [[TDK]] [[Invensense]]<ref name="TDK_Homepage"> TDK InvenSense - Motion Sensors [https://invensense.tdk.com/products/motion-tracking/ TDK InvenSense Website]. Accessed October 26, 2023.</ref> - Major provider for consumer electronics.

*  [[Xsens]] - Specializes in high-precision motion tracking modules often incorporating IMUs.<ref>Yole Développement. "MEMS & Sensors for Wearables Report." 2023.</ref>

*  '''MPU-6050''': A popular low-cost 6-axis IMU (accelerometer + gyroscope) from InvenSense, used in hobbyist projects and early devices like the [[Oculus Rift DK1]].<ref name="MPU6050_Datasheet"> InvenSense Inc. MPU-6000 and MPU-6050 Product Specification Revision 3.4. [https://invensense.tdk.com/wp-content/uploads/2015/02/MPU-6000-Datasheet1.pdf Datasheet Link]</ref>

*  '''MPU-9250''': An InvenSense 9-axis IMU (adds a magnetometer to the MPU-6xxx series capabilities).<ref name="MPU9250_Datasheet"> InvenSense Inc. MPU-9250 Product Specification Revision 1.1. [https://invensense.tdk.com/wp-content/uploads/2015/02/PS-MPU-9250A-01-v1.1.pdf Datasheet Link]</ref> Used in some dev kits and controllers.

*  '''LSM6DSO/LSM6DSOX''': STMicroelectronics 6-axis IMUs used in various HMDs and AR glasses, including the [[HoloLens 2]].

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